Update plaintext_poix transport interface

[chinglee-iot]

• EAGAIN or EWOULDBLOCK should return zero to indicate this send or recv operation can be retried.
• Use error code instead of assert to check the invalid parameters.

Reference this setsockopt document.

If a receive operation has blocked for this much time without receiving additional data, it shall return with a partial count or errno set to [EAGAIN] or [EWOULDBLOCK] if no data is received.

If a send operation has blocked for this time, it shall return with a partial count or with errno set to [EAGAIN] or [EWOULDBLOCK] if no data is sent.

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